Process for obtaining narrow molecular weight distribution in vinyl aromatic mass polymerization system

ABSTRACT

In a process for the mass polymerization of vinyl aromatic monomer wherein vinyl aromatic monomer is polymerized in successive stages of increasing temperature, the improvement which narrows the molecular weight distribution of the resulting polymer comprising incorporating a polymerization inhibiting agent into a polymerizing mass at a point where about 60 to about 95% of monomer is converted to polymer.

This is a continuation of application Ser. No. 373,118, filed Apr. 29,1982, now abandoned, which is in turn a continuation of Ser. No.186,897, filed Sept. 15, 1980, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to vinyl aromatic polymers and, moreparticularly, to improving their physical properties by altering themolecular weight distribution in the manufacture of such polymers.

In a mass thermal process for the polymerization of vinyl aromaticmonomer, especially styrene monomer, the polymer produced has a broadmolecular weight distribution and a low number average molecular weight(Mn) compared to polymers formed by solution or suspension processes.

It has been observed that styrene polymers having an increased numberaverage molecular weight and a narrow molecular weight distribution, asmeasured by the ratio of the weight average molecular weight (Mw) tonumber average molecular weight, show better processabilitycharacteristics when converted to shaped articles. Apparently, a narrowmolecular weight distribution gives a high viscosity at high shear ratewhich leads to a higher extruder output. It is an object of thisinvention to increase the number average molecular weight in vinylaromatic polymers produced by a mass thermal process.

SUMMARY OF THE INVENTION

In a process for the mass polymerization of vinyl aromatic monomerwherein vinyl aromatic monomer is polymerized in successive stages ofincreasing temperature, the improvement which narrows the molecularweight distribution of the resulting polymer comprising incorporating apolymerization inhibiting agent into a polymerizing mass at a pointwhere about 60 to about 95% of monomer is converted to polymer.

BRIEF DESCRIPTION OF THE INVENTION

The process of this invention can be practiced in the production ofvinyl aromatic polymer, preferably styrene polymer. Examples of typicalstyrene polymer produced using this process include styrene homopolymer,sometimes called "crystal polystyrene" and rubber-modified styrenepolymer, sometimes referred to as "high impact polystyrene."

In the process of this invention certain additives are introduced into avinyl aromatic mass polymerization system before completepolymerization. Such additives are polymerization inhibitors and usuallycontain one or more molecular groupings such as nitro, nitroso, quinoid,phenolic, hydroxy and amino. Polymerization inhibitors useful in thisinvention are described in Boundy and Boyer, "The Polymerization ofStyrene" pp. 259-266, incorporated by reference herein. Specifically,useful inhibitors include those which terminate chain growth or reducerate of initiation. Examples of suitable inhibitors includetetraphenyloctatraene, hydrazobenzene, dinitrobenzene, sulfur, aniline,diphenylvinylbromide, hydroquinone, tetranitromethane and p-tert-butylcatechol. Other polymerization inhibitors are discussed in U.S. Pat.Nos. 4,105,506; 4,132,601; 4,132,602; and 4,132,603, incorporated byreference herein. The preferable inhibitor useful in this invention isp-tert-butyl catechol. A suitable inhibitor in this invention, whenmixed in a solution of styrene and toluene having a ratio of one partinhibitor to 9 parts styrene to 90 parts toluene, will lower the amountof conversion of styrene to polystyrene measured at the end of threehours of polymerization at 100° C., compared to polymerization ofstyrene in a solution of 9 parts styrene and 91 parts toluene under thesame conditions.

In the process of this invention inhibitor additives generally are addedin a suitable compatible solvent, such as ethylbenzene or toluene, intoa polymerized vinyl aromatic during or after the last stage ofpolymerization. Typically, such additives are added when about 60 to 95%of monomer is converted to polymer and preferably wnen about 80 to 90%of monomer is so converted. Preferably, additives are injected into apolymerizing mixture before a post reaction heating stage in a mannerwhich produces good mixing. Alternatively, these additives can be addedwith mixing after the final reaction. Additives are incorporated intosuch a polymer system in such amount as to lower effectively the weightaverage molecular weight/number average molecular weight (Mw/Mn) ratioof the final devolatilized polymer. Typically, between about 0.1 andabout 1.5 wt.%, preferably between about 0.5 and about 1.0 wt.%, of suchadditives are so incorporated based on vinyl aromatic polymer.

The process of this invention can be practiced in a continuous or batchmass polymerization system, although a continuous system is typicallyused commercially. In a continuous process, monomer is polymerized as itproceeds through a plug-flow, multiple-stage reactor system. One suchcontinuous mass polymerization process is described in U.S. Pat. No.3,945,976 incorporated herein by reference. Typically, in a continuousprocess a monomer is introduced into a first stage where free radicalpolymerization begins either thermally or by use of a polymerizationinitiator. As polymerization continues, the polymerizing mass is pumpedinto one or more additional reactors in which varyingtemperature-agitation levels are maintained. As the first polymerizingmass travels through the series of reactors, the temperature increaseswhile the agitation rate decreases. Typically, the final polymerizationstage need not be agitated. A continuous process can be simulated by abatch reactor programmed to increase temperature and decrease agitationrate as a function of time.

In the production of rubber-modified vinyl aromatic polymer in acontinuous, plug-flow, multiple-stage system, a solution of vinylaromatic monomer and rubber are polymerized with agitation in multiplepolymerization zones. After the polymerization begins, the systemseparates into two phases. Initially, the rubber in styrene is presentin the larger amount and is the major or continuous phase. As thereaction proceeds and more polystyrene is formed, a phase inversionoccurs whereupon the polystyrene in styrene becomes the continuousphase. At the phase inversion point the system must be agitatedsufficiently to disperse the polystyrene-grafted rubber phase intoroughly spherical particles which act to reinforce an otherwise brittlepolystyrene matrix. Typically, polymerization is continued to a level inthe last reactor stage such that up to about 95 percent of monomer hasbeen converted to polymer, although about 80 to 90% conversion ispreferred. Typically, polymeric material removed from the last reactorstage is devolatilized to remove residual monomer. Sufficient agitationis maintained in the first two reactor stages to disperse rubberparticles adequately within the polymerizing mass. Typically, the laststage need not be agitated. The level of agitation required in aspecific reactor system can be optimized readily by routineexperimentation.

Rubbers which can be used in this invention include polybutadiene (PBD)and styrene-butadiene (SBR) rubbers. Typically useful PBD rubbers arelinear and branched polymers if butadiene containing from 25 to 99percent cis content with less than 20 percent free vinyl unsaturation(i.e., 1,2-addition). A commonly used PBD would contain about 35 percentcis and about 14 percent free vinyl unsaturation. Solution viscositiesfor useful PBD's range from 25 to 220 centipoise and preferably rangefrom 70 to 190 centipoise measured at a concentration of 5 percent byweight in styrene at 30° C. Useful SBR rubbers are random or blockcopolymers of butadiene and styrene, or combinations thereof, with 5 to50 percent bound styrene. Typical solution viscosities are 20 to 190centipoise and typical Mooney viscosities are 30 to 120. These rubberscan be present in styrene polymers at levels from about 2 to 20 percentand typically from about 3 to 10 percent.

Although the preferred polymerization system contains three reactorstages, the number of stages can be varied as long as the sequence oftemperature ranges and agitation substantially is maintained.

In addition to vinyl aromatic monomer and rubber, up to about 10 percentof other materials can be included in the polymerization feedstock, suchas stabilizers, antioxidants, colorants, flame retardants, andlubricants.

This invention is demonstrated but not limited by the followingExamples.

EXAMPLE I

In a batch simulation of a continuous process, styrene was partiallypolymerized in a one-half gallon Chemco stainless steel reactor fittedwith a valve through which molten polymer can be withdrawn, a four-bladeagitator and an internal cooling coil. A feedstock comprising styrenemonomer was polymerized in such a reactor withtime-temperature-agitation cycles being (1) 2.27 hours at 127° C. at 40rpm, (b) 2.27 hours at 142° C. at 10 rpm, and (c) 1.13 hours at 170° C.at 10 rpm. After partial polymerization, product containing about 90.2%polymer and 9.8% monomer was chopped. Two batches of such choppedprepolymer were dry blended and then returned to the reactor togetherwith 1.0 wt.% of t-butyl catechol. The resulting mixture was heated to170° C. over one hour and then heated to 210° C. over one-half hour. AControl run (A) was performed without using t-butyl catechol. Theresults of these experiments are shown in Table I.

                  TABLE I                                                         ______________________________________                                                      Pre-    Control  Example                                                      Polymer A        I                                              ______________________________________                                        Volatiles (% Styrene)                                                         Before devolatilization                                                                       9.8       4.1       7.17                                      After devolatilization                                                                        --         0.028    0.021                                     Conversion (%)  90.2      96       93                                         Melt Flow Rate (g/10 min.)                                                                    --        3.7      3.5                                        Methanol Solubles                                                                             --         0.88     1.40                                      Heat Distortion Temp. (°C.)                                                            --        86       86                                         Vicat Softening Point (°C.)                                                            --        107      107                                        Molecular Weight                                                              --Mn            111,600    79,200   99,100                                    --Mw            286,600   263,100  254,100                                    --Mw/--Mn       2.6       3.3      2.6                                        ______________________________________                                    

EXAMPLES II-III

A series of styrene polymerizations were performed in a continuouspolymerization unit similar to that described in U.S. Pat. No.3,945,976, having three reactors. A solution of t-butyl catechol (33wt.% in toluene) was added to the prepolymer melt stream after the thirdreactor using a Lapp pump calibrated to deliver a desired amount ofadditive. The resulting mixture of styrene prepolymer and additive wasintensively mixed by adding the mixture to the suction side of a gearpump and then transferring the mixture to a devolatilizer. Table IIshows the data from two polymerizations using an inhibitor additive andcontrol data for polymer used in such polymerization without additive.

                  TABLE II                                                        ______________________________________                                                   Control                                                                              Example  Control  Example                                              B      II       C        III                                       ______________________________________                                        Melt Flow Rate                                                                             2.9      2.2      3.0    2.5                                     (g/10 min.)                                                                   Heat Distortion Temp.                                                                       87       82       83     82                                     (°C.)                                                                  Vicat Softening Point                                                                      103      104      104    101                                     (°C.)                                                                  --Mn (× 10.sup.-3)                                                                    77       97       89    104                                     --Mw (× 10.sup.-3)                                                                   275      296      293    310                                     --Mw/--Mn    3.6      3.1      3.3    3.0                                     Residual Styrene (%)                                                                        0.50     0.25     0.36   0.50                                   Methanol Solubles                                                                          1.2      2.1      1.5    1.9                                     (%)                                                                           ______________________________________                                    

EXAMPLE IV

An experiment was performed using a procedure similar to that describedin Example I. A prepolymer was formed by reacting an agitated mixture ofstyrene monomer and polybutadiene rubber (Firestone Diene 35) intime-temperature cycles of (a) 2.27 hours at 127° C.; (b) 2.27 hours at142° C. and (c) 1.13 hours at 170° C. p-tert-Butyl catechol in tenmilliliters of ethylbenzene was added to chopped prepolymer and themixture returned to the reactor and heated with agitation at 10 rpm to170° C. over one hour and to 210° over one-half hour. Results of thisexperiment together with a Control run (D) are shown in Table III.

                  TABLE III                                                       ______________________________________                                                         Control                                                                              Example                                                                D (1)  IV                                                    ______________________________________                                        Styrene (wt. %)    90.0     90.0                                              Rubber (wt. %)     8.0      8.0                                               Mineral Oil (wt. %)                                                                              1.8      1.8                                               BHT (wt. %) (2)    0.2      0.2                                                .sub.-t-Dodecyl Mercaptan (ppm)                                                                 25       25                                                p-tert-Butyl catechol (wt. %)                                                                    --       0.5                                               Melt Flow Rate (g/10 min.)                                                                       2.0      2.4                                               Heat Distortion Temp. (°C.)                                                               79       73                                                --Mn (× 10.sup.-3)                                                                         70.5     82.5                                              --Mw (× 10.sup.-3)                                                                         240      261                                               --Mw/--Mn          3.4      3.2                                               Final Conversion (%)                                                                             90.9     91.9                                              Residual Styrene (%)                                                                             0.16     0.25                                              ______________________________________                                         (1) Prepolymer reacted for additional 1.13 hours at 210° C.            (2) Butylhydroxytoluene  antioxidant.                                    

The data show that addition of a crosslinking agent into a partiallypolymerized vinyl aromatic increases the number average molecular weightand narrows the molecular weight distribution.

What is claimed is:
 1. A process for free radical mass polymerization ofa mixture of vinyl aromatic monomer containing 0 to about 20 wt.%polybutadiene or styrene/butadiene rubber comprising polymerizing suchmixture in successive stages with increasing temperature and agitationand incorporating a polymerization inhibiting agent into the resultingpolymerizing mixture at a point where 60 to about 95% of monomer isconverted to polymer, such inhibiting agent incorporated at a level atwhich polymerization continues, whereby the molecular weightdistribution of the resulting polymer is reduced.
 2. The process ofclaim 1 wherein the vinyl aromatic monomer is styrene monomer.
 3. Theprocess of claim 1 wherein the inhibiting agent is injected into thepolymerizing mass as a solution in a suitable solvent.
 4. The process ofclaim 1 wherein the inhibiting agent is injected into the polymerizingmass at a point where about 80 to about 90% of monomer is converted topolymer.
 5. The process of claim 1 wherein the inhibiting agent isincorporated in a point before a final heating stage.
 6. The process ofclaim 1 wherein the inhibiting agent is selected from the groupconsisting of tetraphenyloctatraene, hydrazobenzene, dinitrobenzene,sulfur, aniline, diphenylvinylbromide, hydroquinone, tetranitromethaneand p-tert-butyl catechol.
 7. The process of claim 6 wherein theinhibiting agent is p-tert-butyl catechol.
 8. The process of claim 6wherein the inhibiting agent is injected into a polymerizing mass ofstyrene at a point where about 80 to about 90% of monomer is convertedpolymer.
 9. The process of claim 1 wherein the inhibiting agent is addedin an amount of about 0.1 to about 1.5 wt.% of the polymerizing mass.10. The process of claim 1 wherein the polymerizing mixture contains 2to about 20 wt.% rubber.